1. Field of the Invention
The present invention relates to an idling-engine-speed control method and an idling-engine-speed controller.
2. Description of the Prior Art
As disclosed in, for example, the Japanese Patent Publication No. 53544/1986, a conventional engine-speed controller stabilizes an idling engine speed under the transition state from cold state to hot state of an engine by controlling an intake air quantity in accordance with the deviation between a target engine speed set correspondingly to the cooling-water temperature of the engine and the actual speed of the engine. In this case, a cooling-water-temperature signal output from a water-temperature sensor for measuring the cooling-water temperature of the engine is A/D-converted by the input port of a microcomputer and transferred to a CPU for computing the controlled variable of idling engine speed. That is, the digital value obtained by A/D-converting a voltage input from the above water-temperature sensor is used as a cooling-water temperature to be referenced to select the above target engine speed.
An actual water-temperature change gets larger than the change of the A/D-converted value in a high temperature region because of the characteristic of a water-temperature sensor. Therefore, the A/D-converted value of a cooling-water temperature (cooling-water temperature to be referenced to select a target idling engine speed) may sensitively react even for the change of outputs of an water-temperature sensor in minimum resolutions around a warming-up completed water temperature. Therefore, in the case of a conventional idling-engine-speed control method, a cooling-water temperature greatly fluctuates even if a slight change occurs in outputs of a water-temperature sensor immediately before completion of warming-up and as a result, a problem occurs that a target engine speed and a bypass air quantity to be determined by the cooling-water temperature also fluctuate and thereby, the engine speed immediately before completion of warming-up is not stabilized. Moreover, because the rise rate of an actual cooling-water temperature is loosened as the actual cooling-water temperature approaches the cooling-water temperature at completion of warming-up, a delicate state that cooling-water temperature values are changed or not changed by 1 bit easily continues and thereby, slight fluctuation in outputs of the water-temperature sensor easily occurs for a long time. Furthermore, in the region from the time immediately before completion of warming-up to the time of completion of warming-up, the gradient of a target ending speed and a bypass air quantity to be set correspondingly to a cooling-water temperature to the cooling-water temperature relatively increases. Therefore, when the above cooling-water temperature value fluctuates, set values of the above target engine speed and bypass air quantity greatly fluctuate and hunting may occur because the engine speed immediately before completion of warming-up.